1. Field of the Invention
The present invention relates generally to an anti-skid brake control system for an automotive brake system. More specifically, the invention relates to an anti-skid brake control system including a technique for deriving a projected vehicular speed with an improved precision level which is satisfactory for establishing precise anti-skid control.
2. Description of the Background Art
It is well known that vehicular braking performance becomes optimum when wheel slippage becomes a certain value, e.g. 10% to 20%. Therefore, as is well known, anti-skid or anti-lock brake control is generally performed for maintaining the wheel slippage within a predetermined optimal range. For this, wheel slippage is monitored for controlling operational modes of a vehicular braking system for increasing braking pressure in APPLICATION mode, holding braking pressure constant in HOLD mode and decreasing braking pressure in RELEASE mode. In the vehicular anti-skid brake control, the APPLICATION mode is selected in normal operational condition for allowing linearly decelerating the vehicle according to increasing of the braking pressure. Anti-skid control is initiated upon detection of wheel deceleration during braking operation, in a greater magnitude than a preset deceleration threshold to switch operational mode of the brake system from APPLICATION mode to HOLD mode. Therefore, at the initial stage of an anti-skid brake control cycle, the braking pressure is held constant at an increased pressure at which the wheel deceleration increased across the wheel deceleration threshold is obtained, in HOLD mode. Because of the increased pressure in this HOLD mode, wheel speed further decelerates at a greater rate than deceleration of the vehicle speed. Therefore, wheel slippage which represents a ratio of difference of the vehicle speed and the wheel speed versus the vehicle speed, becomes greater than a target speed which represents the optimal wheel speed for obtaining optimum vehicular braking characteristics. When the wheel slippage becomes greater than a wheel slippage threshold and thus the wheel speed becomes decreased across the target speed, operational mode of the brake system is again switched into the RELEASE mode for decreasing the braking pressure for resuming the wheel speed toward the target speed. By this, the wheel speed resumes across the target speed. Therefore, wheel acceleration increases across a preset acceleration threshold. Then, the mode is again switched into the HOLD mode. Because of decreased braking pressure, the wheel speed overshoots to increase across the vehicle speed and then decelerated to the vehicle speed. Accordingly, the wheel acceleration again decreases across the acceleration threshold. Then, mode is switched to APPLICATION mode again.
In the anti-skid control set forth above, it becomes necessary to monitor the vehicle speed for deriving the wheel slippage. It is possible to directly measure the vehicle speed by means of an appropriate sensor, such as a doppler sensor. However, such sensor is unacceptably expensive and therefor is not practical for use in the anti-skid control system in view of the cost. Therefore, it is usual way for monitoring the vehicle speed in anti-skid control to latch a wheel speed upon initiation of anti-skid control as vehicle speed representing data, because upon initiation of the anti-skid control where the wheel deceleration increased across the wheel deceleration threshold, is approximately coincident with the vehicle speed. This vehicle speed representing data will be hereafter referred to as "projected speed". Based on the latched value, a vehicle speed represented data is projected utilizing a given vehicle deceleration indicative gradient which can be derived in various ways.
In another approach, the projected vehicular speed representative data has been derived on the basis of a longitudinal acceleration exerted on the vehicular body. In case that the longitudinal acceleration is used as a parameter for deriving the projected vehicular speed representative data, slop on the road can influence for monitoring longitudinal acceleration as will be naturally appreciated. For instance, the vehicle is in hill-climbing, the forward acceleration can be smaller than that of actual value due to influence of backward component of gravity force. On the other hand, when the vehicle is in down-hill driving condition, the forward acceleration can be greater than that of the actual value due to influence of the forward component of gravity force. In order to avoid influence of such gravity force in measurement of longitudinal acceleration for improving accuracy in derivation of the projected vehicular speed representative data, Japanese Patent Second (examined) Publication (Tokko) Showa 48-27710 proposes correction of the measured longitudinal acceleration value with a correction value which is derived on the basis of a road slop angle indicative data obtained immediately before initiation of vehicular braking operation.
Such prior proposal is not satisfactory because it cannot follow slop angle variation during vehicular braking operation. For instance, when the braking operation is performed on the flat road condition and the vehicle then enters into slopped road, correction value derived with respect to flat road may not satisfactorily compensate the gravity force component.
Additionally, in either case, the gradient data as well as the projected vehicular speed representative data may contain error even after improvement of the precision level. Such error may be accumulated through operation to cause malfunction in anti-skid control.